Dicarboxylic acids are useful as chelating agents, detergent builders, and emulsifying agents. These acids, being composed of only the elements C, H and 0, do not pose the environmental problems that other compounds containing heteroatoms such as N, S, and P pose. The alpha,omega-dicarboxylic acids can be prepared in a two-step process by first reacting a glycol with ethylene oxide and an alkaline catalyst and thereafter converting the polyoxyalkylene glycol to a polyoxyalkylene-alpha,omega-dicarboxylic acid.
Japanese Patent No. 50-96516, issued July 31, 1975, discloses a process for the preparation of carboxylic acid salts by the liquid phase dehydrogenation of alcohols with caustic alkali in the presence of precious metal catalysts, including palladium. This process uses a relatively high temperature, 100.degree. C.-270.degree. C. These high temperatures can degrade the ether linkages especially in the highly ethoxylated alcohols.
It is known to use nitroxyl radicals/oxoammonium salts in the oxidation of primary alcohols to produce aldehydes and acids and secondary alcohols to ketones. Journal of Organic Chemistry, vol. 52 (12), pp. 2559-2562; Pure and Applied Chemistry, vol. 62(2), 1990, pp. 217-222; Journal of Organic Chemistry, Vol. 55, 1990, pp. 462-466. The primary products produced in these processes are aldehydes and the stoichiometrically consumed oxidant is hypochlorite.
It is generally more difficult to oxidize polyoxyalkylene glycols than alkanols as it is difficult to oxidize polyoxyalkylene glycols without splitting the molecular chain at the ether linkage thereby producing a large proportion of undesired by-products.
It is therefore an object of this invention to produce polyoxyalkylene-alpha,omega-dicarboxylic acids in high yields and with high selectivities from polyoxyalkylene glycols without producing large amounts of other products such as oxalic acid and glycolic acid.
It has been found that polyoxyalkylene-alpha,omega-dicarboxylic acids can be produced in high yields and with high selectivities by using a stable free radical nitroxide and a NO.sub.x -generating compound and an optionally, oxidant and/or a solvent.